Display tessellation plans (2/4): size rules and the facet angle (#803)

Per-face target side lengths, the caller's facet angle, refined grid lines, and fixes for underflowing coordinates and straight edges ending exactly on their vertex. Nothing drawn changes yet.

Refs #303

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -3365,6 +3365,7 @@ name = "solidcraft-kernel"
 version = "0.8.0"
 dependencies = [
  "log",
+ "rayon",
  "serde",
  "serde_json",
  "solidcraft-geom",